The present invention relates to a polarization beam splitter and a laser interferometry length measuring apparatus, and more particularly, to a polarization beam splitter suitable for causing two beams separated by the polarization beam splitter to be in parallel each other and to a laser interferometry length measuring apparatus employing the polarization beam splitter.
In the laser interferometry length measuring apparatus, a gas laser or a semiconductor laser is used as a light source, and after its beam is separated into a reference beam and a length measuring beam, a beam reflected from an object whose length is to be measured is combined again with the reference beam, and an interference fringe and a beat thus caused by the combination thereof are observed for obtaining length measurement (displacement) information.
The construction of such laser interferometry length measuring apparatus is based on interferometer of Michelson type shown in FIG. 5. To be more concrete, a beam from light source 20 is split by polarization beam splitter 21 into a reference beam and a length measuring beam, and the length measuring beam reflects on movable mirror 22 attached on an object whose length is to be measured, and then is combined, at polarization beam splitter 21, with a reference beam reflected on fixed mirror 23 to generate interference fringes. In this case, there are two systems; one is an interference fringes counting system that counts movement of interference fringes generated by the returning beam and the other is a heterodyne system wherein coherent plural wavelengths are used for generating beats, and frequency changes of beats are detected by utilizing the phenomenon that the frequency of the length measuring beam is Doppler-shifted depending on the speed of the object whose length is to be measured (see PRECISION ENGINEERING Vol. 1, No. 1 (1979) 85, PRECISION ENGINEERING Vol. 5, No. 3 (1983) 111).
When splitting into the reference beam and the length measuring beam at a beam splitter as described above, the reference beam and the length measuring beam are separated in the most simple way by the beam splitter so that they may meet at right angles each other to irradiate respectively the fixed mirror and the movable mirror as shown in FIG. 5. However, when an optical path is provided almost straight, both the reference beam and the length measuring beam are needed to go out in parallel each other. In this case, the beam splitter and a reflection plate have been combined so that the reference beam and the length measuring beam both are in parallel may be obtained.
In FIG. 6, polarization beam splitting surface 31 is provided on a joint surface between triangle prism 32 and triangle prism 33, and triangle prism 35 having thereon a reflection surface (or a total reflection surface) 34 is positioned so that the above-described reflection surface 34 and polarization beam splitting surface 31 may be in parallel each other, thus, a beam enters or leaves a surface of each of prisms 32, 33 and 35 at a right angle. A beam from a light source entering the above-described polarization beam splitting surface 31 at an angle of 45.degree. is split, for example, to be a reference beam that is a transmitting beam and a length measuring beam that is a reflected beam, and the above-mentioned length measuring beam that advances in the direction meeting at a right angle with aforementioned reference beam is caused to reflect and refracted on the aforesaid reflection surface 34, in the direction that is in parallel with the above-described reference beam.
Incidentally, the mark " " and the mark ".circleincircle." in FIG. 6 show respectively the directions of polarization meeting at a right angle each other.
However, when making both a reference beam and a length measuring beam to be in parallel as described above, even when polarization beam splitting surface 31 and reflection surface 34 are precisely arranged to be in parallel each other, the reference beam and the length measuring beam can not be in parallel precisely and thereby the reflected beam can not be combined precisely if a vertical angle of each triangle prism is not accurate and thereby beam-leaving surfaces on triangle prisms 32 and 35 can not be in parallel precisely.
Further, in the above-described construction, an optical element composed of triangle prisms 32 and 33 and an optical element composed of triangle prism 35 need to be positioned precisely even if a vertical angle of each triangle prism is accurate. When such positioning is not accurate, polarization beam splitting surface 31 and reflection surface 34 can not be in parallel precisely, thus, parallelism of leaving beams after their separation is deteriorated.
In this connection, when prism 36 whose lateral section is a parallelogram and triangle prism 37 are used in combination as shown in FIG. 7, polarization beam splitting surface 31 and reflection surface 34 can be in parallel stably if parallelism of end faces is secured in the course of making prism 36. However, a beam reflected on a polarization beam splitting surface (a length measuring beam) and a transmitting beam (a reference beam) leave end faces of different prisms respectively. Therefore, parallelism of both end faces needs to be accurate, and it has been difficult to obtain stably the separated beams which are in parallel each other, due to parallelism of joint surfaces and accuracy of a vertical angle of each triangle prism both of which can not be precise constantly.
The present invention has been devised in view of the aforesaid problems, and its first object is to provide a polarization beam splitter wherein there are used prisms in the form of parallel plates whose parallelism can be obtained relatively easily compared with vertical angle accuracy of a triangle prism, and thereby two beams generated through separation on a polarization beam splitter can be caused to be parallel beams which are accurate in terms of parallelism without any requirement for high level accuracy of a vertical angle of each triangle prism and of joint thereof. The first object in to further provide a laser interferometry length measuring apparatus wherein the specific characteristics of the above mentioned polarization beam splitter are utilized.
Further example of the constitution of a laser interferometry length measuring apparatus of a type of counting number of interference fringes is shown in FIG. 13. To be concrete, a beam from stabilized laser light source 130 is separated by beam splitter 131 into a reference beam and a length measuring beam, and the length measuring beam is reflected on movable corner cube (a movable mirror) 132 attached on an object whose length is to be measured, then, the reflected length measuring beam is combined with the reference beam reflected on fixed corner cube (a fixed mirror) 133 on the beam splitter 131 to generate an interference fringe. (See page 108 and others in Vol. 26 No. 2 (1988) of Optical and Electro-optical Engineering Contact).
For the detection of intensity of interference fringes caused be combined light including a reference beam and a length measuring beam in above-described laser interferometry length measuring apparatus, or for the detection of beat signals of interference fringes caused by two-frequency light source, a polarization detection optical system is usually used for optical phase separation so that both amount and direction may be observed. Each of beams thus phase-separated is subjected to photoelectric conversion conducted by a photoelectric converter composed of a photodiode or the like, thus the intensity of interference fringes or beat signals of interference fringes are detected.
In FIG. 13, a reference beam that goes to fixed corner cube 133 and returns from it and then is combined with a length measuring beam is caused to go to wavelength .lambda./8 phase plate 134 and return from it so that the reference beam may become circularly polarized light, while, a length measuring beam that goes to movable corner cube 132 and returns from it is caused to remain as linearly polarized light, thus the S/N ratio of signals can be improved. In this case, length measuring signals of three phases (0.degree., 90.degree. and 180.degree.) which are different each other by 90.degree. due to angular positions of polarization plate 135 around an optical axis are obtained.
In addition to the foregoing, there are other methods of polarization detection optical system wherein interference fringe intensity signals of two phases differing each other by 180.degree. are obtained as a transmission beam or a reflection beam by positioning a polarization beam splitter to be inclined by 45.degree. against an optical axis, and signals of three phases or four phases differing each other by 90.degree. are obtained by causing a beam transmitted through a .lambda./4 phase plate to enter one of two polarization splitter, or signals whose phase is shifted by 90.degree. are obtained by shifting a phase through a phase plate of .lambda./4 wavelength by 90.degree. and then extracting 45.degree. components of a reference beam and a length measuring beam.
In a polarization detection optical system, it is possible to identify the direction of change in phase of interference fringes if signals of two phases (sin .theta., cos .theta.) differing each other by 90.degree. can be obtained. In the example shown in FIG. 13, however, signals of three phases (0.degree., 90.degree. and 180.degree.) differing each other by 90.degree. are generated purposely in an optical system for the purpose of restraining an influence of the change in the intensity level of interference fringes caused by a deviated optical axis or by flickering air, and signals of three phases thus generated are converted photoelectrically by photoelectric conversion element 136 such as a photodiode, and thereby signals of two phases (sin .theta., cos .theta.) differing each other by 90.degree. are generated by obtaining difference between two phases by means of subtracter 137, thus, the intensity components changed commonly by the above-described deviated optical axis can be eliminated.
Incidentally, in FIG. 13, the numeral 138 is a filter, 139 is an amplifier and 140 is a polarization beam splitter.
When signals of plural phases differing each other obtained by the polarization detection optical system are received by the photoelectric conversion element, the sectional area of a beam is larger than the area of light-receiving section because it is necessary to establish an allowable value for a tilt of the beam. In this case, when concentricity of the reference beam and the length measuring beam is excellent enough, there is no problem. However, when the concentricity of the reference beam and the length measuring beam is slightly deteriorated and thereby plural interference fringes appear within the detection beam as shown in FIG. 14, phasic relation of three phases gets out of order, resulting in the deterioration of measurement accuracy unless relative position of the light-receiving section of the photoelectric conversion element which converts photoelectrically the 3-phase signals after the aforesaid polarization against the beam is precisely adjusted.
Adjustment of the relative position of the light-receiving section of the photoelectric conversion element against the beam is very difficult. In the case of separation into three phases each differing by 90.degree. shown in FIG. 13, when concentricity of the reference beam and the length measuring beam is changed, phases of 3-phase signals which should be detected at an interval of 90.degree. are changed. Therefore, the phasic relation of 2-phase signals after taking a difference can not be 90.degree. precisely, resulting in a problem for practical use.
When the axis of abscissa represents cos .theta. and the axis of ordinate represents sin .theta. for 2-phase signals representing signals which are divided into three phases differing in terms of a phase by 90.degree. each other and from which a difference has been taken, a Lissajous's figure takes a form of a circle as shown in FIG. 15, and its rotation direction, such as a clockwise rotation or a counter clockwise rotation, tells a direction of a phase change of interference fringes, close or far direction.
However, when the above-mentioned 2-phase signals are not harmonious in terms of a phase, the above-described Lissajous's figure does not take a circle as shown in FIG. 16 but it takes an oval whose major axis (minor axis) does not exist on X-axis and Y-axis. In this case, when a high measurement accuracy is kept through interior division of a phase of one circumference 2.pi. of the Lissajous's figure, the movement on the Lissajous's figure shows non-uniform motion and non-circle (oval) even if the length measurement change is uniform. Therefore, when the interior-divided length measurement values are read from a locus of the Lissajous's figure, it is not possible to avoid a cyclic length measurement error wherein one rotation is one cycle. In addition to that, the form of an oval is very difficult to be corrected by delaying electrically the phase, because the range of signal velocity is broad to be from DC--several MHz.
Namely, the phase relation (90.degree.) of final 2-phase signals (sin .theta., cos .theta.), when it is not secured fully in the first optical system, tends to deteriorate the linearity of length measurement accuracy. This is a primary error related to the linearity of a laser interferometry length measuring apparatus corresponding to the intensity change of one cycle (2.pi.) of interference fringes, and the second and tertiary errors with frequencis of .pi. and .pi./2 . . . are further assumed. However, it is possible to solve problems in terms of accuracy by eliminating the primary error, because the actual non-linearity is mainly caused by the primary error caused by one occasion of reflection of unnecessary light.
Further, when verifying a phase relation of 3-phase signals (0.degree., 90.degree. and 180.degree.) referring FIG. 9, there hardly is movement of phase difference of 180.degree. by means of a polarization beam splitter between the first phase and the third phase (0.degree., 180.degree.), but the 90.degree. phase shift by means of a phase plate for the second phase of 90.degree. position needs to be conducted accurately. Therefore, there is possibility that a phase difference is easily changed by the deviation of an optical axis that is caused by mechanical variation. In this case, when the phase of the second phase is deviated to the first phase by .DELTA..theta. as shown in FIG. 17, mutual errors of about .DELTA..theta..sup.2 take place on difference signals between the first phase and the second phase and difference signals between the second phase and the third phase, as shown in the following equations. ##EQU1##
As described above, when obtaining 2-phase signals by obtaining a difference of 3-phase signals differing in terms of a phase by 90.degree. in the optical system for detection of polarization light in a laser interferometry length measuring apparatus, the phase of the first 3-phase signals obtained is required to be accurate, and in particular, the phase deviation caused by the deviated optical axis of the second phase (90.degree.) that is obtained among the above-described 3-phase signals through the transmission of a phase plate has been a problem. Further, as stated above, the relative position of a photoelectric transfer element against a beam of each phase after separation has been needed to be accurate.
The invention has been devised in view of the situation mentioned above, and its second object is that an optical axis of each of 3-phase signals among four phases can be obtained accurately as a parallel beam and a phase angle deviation can be corrected optically in a polarization light detecting optical system to be used for a laser interferometry length measuring apparatus wherein four-phase splitting is especially possible due to two sets of polarization beam splitters.